The present invention relates to calibration of electronic circuits, and in particular, to circuits and methods for calibrating the frequency response of a filter.
Some electronic circuits use reactive elements to provide a frequency response. For example, analog filters may have inductors and/or capacitors which react to the frequencies of a signal to provide corresponding frequency responses. In the case of a low pass filter, the capacitor may be coupled to an input signal through a resistor. As a frequency of the signal increases, the capacitor may change impedance and provide attenuation. At a characteristic corner frequency, the signal may be attenuated to half its original power. Signals with frequencies much higher than the corner frequency may be attenuated such that the signals are essentially removed from the output signal.
Analog filters remove unwanted portions of signals. Analog filters include low pass, high pass, and band pass filters. Depending on the application, a different filter type may be implemented. For example, communication systems have circuits which mix signals together in order to modulate and demodulate. The mixers provide extraneous signals which can be removed by an analog filter.
Many communication systems include digitally programmable filtering as well as analog and digital signal processing. The frequency response of the filter can be modified by altering some of the component values in the analog filter circuit. For example, for frequencies between 100 khz and 100 Mhz, a low pass filter include capacitors that may be digitally controlled to provide a range of frequency adjustment, for example.
Accurate adjustment of digitally programmable filters affects system performance. However, many times the capacitors or other filter components may exhibit a significant spread in value. For example, a metal-oxide-metal (MOM) thin-film capacitor and poly resistors may be manufactured on the same integrated circuit and exhibit an overall tolerance beyond 30%. Acceptable performance for a communication system may require a 1% tolerance.
Traditional methods of calibrating the frequency response of a filter typically require specialized analog circuits. However, such analog circuits take up large die area and may not provide the calibration accuracy desired. Calibration implemented during final testing may be expensive and may not take into account the variances of values due to drift or temperature during normal operation.